This is the 35 U.S.C. §371 National Stage of PCT/JP2008/066343 filed Sep. 10, 2008.
The present invention relates to a method for producing an electroconductive polymer electrode, and a dye-sensitized solar cell equipped with the electroconductive polymer electrode.
In recent years, various kinds of solar cells have been proposed as a photoelectric conversion device converting light energy to electric energy. Among these, a dye-sensitized solar cell, which was introduced by M. Gratzel, et al. (Ecole Polytechnique Federale de Lausanne, Swiss) on 1991 (Nature, vol. 353, pp. 737-740 (1991)), is expected to be practiced owing to such advantages that the materials are inexpensive, and the solar cell can be produced in a relatively simple process.
A dye-sensitized solar cell is generally constituted by a semiconductor electrode having a photoelectric conversion layer containing a semiconductor in which a dye is absorbed on an electroconductive substrate, a counter electrode (catalyst electrode) containing an electroconductive substrate having a catalyst layer formed thereon provided to face the semiconductor electrode, and an electrolyte layer (charge transporting layer) retained between the semiconductor electrode and the counter electrode.
The catalyst electrode of the dye-sensitized solar cell is required to have such electrode characteristics that the reduction reaction of reducing an oxidant of a redox pair (for example, I3−/I−) to a reductant (for example, the reduction reaction of reducing I3− to I−) proceeds immediately.
In general, examples of the catalyst electrode of the dye-sensitized solar cell include a platinum electrode, and examples of a production method thereof include a vapor deposition method, a sputtering method, and a method, in which a precursor of platinum is coated on an electrode by a dipping method, a spraying method or the like, and then heated.
However, platinum is an expensive noble metal, and in the case where a platinum electrode is produced by a vacuum deposition method or a sputtering method, a large loss of the material occurs to lower the productivity, and a vacuum equipment is necessarily provided to increase the equipment expenses, which brings about an increased production cost. Furthermore, it has been known that platinum that is vapor-deposited is dissolved in an iodine electrolytic solution in the presence of oxygen, and thus the use thereof involves a problem in safety. The dipping method and the spraying method are conveniently practiced since a vacuum equipment is not used, but the methods require baking at around 400° C. in the step subsequent to coating on the substrate, and thus involve such a problem that the methods cannot be applied to a substrate that is weak against heat, such as a resin substrate.
As a substitute of the platinum electrode, an electroconductive polymer catalyst electrode using poly(3,4-ethylenedioxythiophene) (which is hereinafter abbreviated as PEDOT) as an electroconductive polymer excellent in stability and electroconductivity has been reported.
Non-patent Document 1 discloses a dye-sensitized solar cell using an ionic liquid electrolyte and a PEDOT counter electrode. The document discloses a production method of the PEDOT counter electrode, in which such a process is repeated that a dispersion aqueous solution of particles of PEDOT having polystyrenesulfonic acid (PSS) as a dopant (which is hereinafter abbreviated as PEDOT-PSS) is formed into a film by spin-coating on electroconductive glass, and the film is dried and then heat-treated.
However, the electroconductive polymer catalyst electrode formed from the dispersion aqueous solution of PEDOT-PSS particles has low electroconductivity, and thus the capability thereof is lower than the platinum electrode in the case where an electrolytic solution containing an organic solvent is used.
Under the circumstances, Patent Document 1 discloses a dye-sensitized solar cell using a hole collecting electrode (catalyst electrode) formed of an organic film (PEDOT-PTS) formed simultaneously with polymerization of a monomer of PEDOT with p-toluenesulfonic acid (PTS) as a dopant. According to the document, a hole collecting electrode can be produced in a simple process at low cost as compared to a conventional method for forming a catalyst electrode, and a dye-sensitized solar cell can be provided with advantages in production process and production cost, while the solar cell maintains a cell capability that is equivalent to a dye-sensitized solar cell using a conventional catalyst electrode using platinum.
However, the PEDOT electroconductive polymer electrode disclosed in Patent Document 1 has a cell capability that is equivalent to a dye-sensitized solar cell using a conventional catalyst electrode using platinum, but the comparison is performed under a condition of a device conversion efficiency of approximately 5%, and it is unclear as to whether or not the cell can exhibit a comparable capability under a higher capability condition (i.e., a condition where a larger electric current flows). Furthermore, there is a description “an organic film formed simultaneously with polymerization of a monomer”, but a desirable production method is not disclosed. The production method disclosed as an example for the hole collector is only a method of spin-coating a solution containing a monomer, and then polymerizing under heat treatment. The production process is complicated due to the use of a spin coating method, and thus is not necessarily sufficient as an industrial production method. In a spin coating method, patterning for forming devices is difficultly performed upon practical application, and the solution use efficiency is low in the coating operation.
In general, examples of a thin film coating method other than a spin coating method include a printing method, such as a screen printing method and a slit coating method. Upon applying the coating methods, in general, a binder, such as ethyl cellulose, is often added for enhancing the coating property and for enhancing the adhesiveness to a substrate. However, the counter electrode of the dye-sensitized solar cell necessarily has high electroconductivity, and the addition of an ordinary binder lowers the electroconductivity of the total electrode, whereby it is difficult to produce a counter electrode exhibiting sufficient capability with a system having a binder or the like added thereto.
The formation method of the PEDOT electroconductive polymer electrode disclosed in Patent Document 1 is a method disclosed in Patent Document 2, in which a polymerization controlling agent for reducing the polymerization rate is added to a solution containing a monomer and an oxidizing agent, which is formed into a film at room temperature where polymerization difficultly occur, and then subjected to reaction under heat, thereby forming an electroconductive polymer film. As the polymerization controlling agent, the use of imidazole, dimethylsulfoxide or the like has been reported. However, the polymerization reaction proceeds within several minutes to several hours at room temperature even though the polymerization controlling agent is used, and it is difficult to use the solution for a prolonged period of time, thereby providing a problem upon applying to a coating method requiring certain stability of a coating solution.
As having been described above, an electroconductive polymer counter electrode for a dye-sensitized solar cell is required to be excellent in patterning property, use efficiency of a coating solution, and coating reproducibility, and to be high in electroconductivity and catalyst capability, but it has been difficult to satisfy all the requirements.    Patent Document 1: JP-A-2003-317814    Patent Document 2: EP 0,615,256 A    Non-patent Document 1: The 72nd Meeting of Electrochemical Society of Japan, Abstract, p. 471 (2005)